What is Blistering?
Don’t Get Burned with your Paper Choice

You are running a heavy coverage job with a coated sheet or board on a heatset web offset press when the press crew notices an effect on the surface of the printed sheet. The effect they sees appears as a bubble on many parts of the printed sheet. What is this effect? It is known as “blistering”. Okay, now we have the word but what does it mean? Just like a blister that you might get from wearing a pair of ill-fitting shoes, there is also a phenomenon in heatset offset printing that produces a similar effect; only this time the culprit is not a shoe, but the relationship between paper, coatings and heat. The image shown displays the blistering effect.

Now that we know what a blister is, what causes it? The ink is dried in the heatset process by passing the printed paper through the heatset drier. These driers run heat temperatures in the 350 degree range depending upon the press size. This means the paper temperature upon exiting the oven can reach approximately 250 degrees. The heat in the oven evaporates the solvents contained in the ink. Once the paper exits the oven, the ink is semi-fluid. To fully dry the ink the paper passes over “chill rolls” which are rolls with cold water passing through them. These “cold” rolls “shock” and set the ink. This is a “thermoplastic” effect.

With that in mind, how does blistering occur? As the paper is passing through the oven any extra moisture in the paper starts to evaporate due to the high temperatures. The coatings on the paper are absorptive. With the ink on the surface of the sheet, the coating is much less absorptive as the pores of the coating are filled with ink. The moisture will try to escape the paper as it evaporates under the high temperatures. Because of the absorptive nature of the coating most of the moisture can evaporate; however, when ink coverage (especially heavy coverage) is high, ink has “sealed” the surface. Heavy ink coverage “plugs” the pores of the coating rendering it far less absorbent. The moisture is trapped by this sealing and cannot escape. The moisture will then push upward and “pop” the coating from the fiber layer in the sheet. The result is the blister effect. These blisters are the result of the rapid vaporization o moisture when the web is heated. Bilistering should not be confused with delamination, which is a similar effect but caused by something entirely different.

How do you overcome this blishtering problem?

• Lower the oven temperature.
• Speed up so the paper has less time in the oven.
• Install a preheater in the press infeed to even out the papers moisture.
• If the drier is a multistage drier, turn down the line burners or reduce their impingement on the web. Raise the temperature of the circulating air to produce drying.

Extensive testing has been done on Carolina C2S Web Cover to ensure blistering will not occur. Proper moisture levels and heat resistant binders are use to assure both excellent print and press performance.